AN is manufactured in many particulate forms: prills, flakes, powder and granules, as well as saturated aqueous solutions. AN particulate matter is a readily available low-cost source of oxygen and is stable enough at room temperature scale to be safely transported, stored and blended with other explosive components. Given these attributes, it is extensively used in the explosives industry as the main component (in terms of formulation content) for almost all commercial explosives: Ammonium Nitrate-Fuel Oil (ANFO), emulsions and watergels, used in the world today.
Despite the large-scale utilization of AN particulate matter, the salt exhibit a number of undesirable properties. The hygroscopicity, the strong dependence of its solubility with temperature and the specific volume variations ascribed to crystalline transitions, frequently cause problems in production, handling and storage operations.
It is well known that AN particles have a noticeable tendency to cake with storage time. In the absence of transitions on the crystal structure, this phenomenon is related to the adsorption of water and subsequent AN dissolution to form saturated AN solution, which if located at interparticle contact points may lead to crystal bridge formations as it crystallizes with a decrease in temperature. A second mechanism involves the merging of capillary forces due to the presence of solution at the contact points. This is, there is an inter-particle bonding tendency which extrapolated to bulk AN material might lead to cake formation, whose extension, in terms of hardness and quantity of the bulk AN involved in the caking process depends on storage time, storage conditions (pressure, Relative Humidity (RH) and temperature) and product characteristics.
Additionally, this bonding tendency, also known as cohesiveness of the AN material, has an impact on its flowing behaviour. This flowability of bulk AN particles is a critical characteristic, especially if product is used for explosive production. Most of the mechanical blending operations involved in explosive manufacturing require bulk AN particles flowing free, i.e. mass-flow behaviour, thus deviations/restrictions to flow can cause alterations in final explosive formulation. As an example of this problem, when AN particles discharge from hoppers to feed blenders (i.e. mixing with fuel-oil for ANFO production or mixing with emulsion or watergel explosive matrices for “blended explosives”), arching and ratholing are recurrent events at the hopper that results in irregular or even total suppression of AN flow, leading to important operational problems as erroneous explosive compositions.
A comprehensive work has been performed till date with the aim of minimizing particulate AN products cohesiveness, most of it focussed on coating AN particles with different anticaking agents. U.S. Pat. No. 4,001,378 describes anticaking compositions consisting on alkene sulphonates combined with kieselguhr or inorganic powders such as magnesium carbonate or clay. Sprayable aqueous naphthalene sulphonates, which could be mixed with alkene sulphonates, were proposed in U.S. Pat. No. 4,717,555 as anticaking coating. Patent Application EP-A-692468 proposes the use of sprayable compositions containing oily products and waxes together with compounds obtained by reacting an amine or an alcohol with an acid or a carboxylic anhydride containing a C20-C500 hydrocarbonated group, compounds that are known and available in the market.
The state of the art lies on using organic nature active anti-caking agent coatings which act as barrier for minimizing contact area involving direct AN interaction in between adjacent particles, where interparticle bonds can be generated due to capillarity or to crystal bridges after recrystallization.
However, apart from active anti-caking formulation, the coating agent distribution among the AN particles surface will have an impact on its performance. Coating is normally applied by spraying the melted anti-caking over AN particles and its dose is limited by regulation to <0.2 wt % as carbon. The coating system configuration, noozles and drum, is designed to maximize the dispersion of the coating agent. However, the surface structure of the AN particles, if rough enough, could physically impede its dispersion throughout. This is especially evident for porous AN particles produced from prilling AN solutions containing water (1-10 wt %). The moisture present in the prilled AN as AN solution is removed in drying steps, where the smooth surface achieved during crystallization due to the exposure to cooling air at the prilling tower is lost, due to the merging towards the surface of part of the inner AN solution through the pore network and its subsequent crystallization at the surface resulting in the formation of surface bulges.
Most commercially available coating technologies claim to provide anticaking properties and even moisture barrier properties, connected to its hydrophobicity. However, as it is known by the person skilled in the art, all the commonly used organic coatings do not show specific activity as moisture traps and tend to be poor water vapour barriers. This together with relatively poor coating dispersion (i.e. normally far from total coverage of AN particles surface) results in direct AN contact areas in between particles susceptible to get moisturized and thus to form inter-particle bonds.
U.S. Pat. No. 5,472,530 claims the application of aqueous solutions of magnesium or calcium nitrate in process stage prior to final drying step as anticaking agent. Then the particles are dried to a water content equal or below the maximum amount of crystal water that can be bound by said inorganic salts. In contrast to other coating additives, these partially hydrated salts work as moisture traps. However, it was found that for porous AN products this solution is not well suited due to the fact that it would induce structural changes such as pore blocking, affecting its final characteristics as raw material for explosive manufacture.
In a similar basis, but with the objective of stabilizing AN particles against thermal cycling, U.S. Pat. No. 4,486,396 claims the coating with porous powders, preferably silicon dioxide and a dust-biding agent and/or an anticaking agent, showing this coating the ability to bind water. However, operational difficulties such as generation of dusty environments and the need to fix powder material to AN particle surface limits this technique. Even mechanical mixing with drying agents, such as silicagel and AN particles containing Magnesium Nitrate (MgN) has been claimed, EP 1 123 257, as an effective stabilizing additive against thermal cycling if its dose drying capacity accounts for the free water present at the AN.
In spite of this background, there is a continuing need and demand for ammonium nitrate products showing improved flowing behavior that complement or improve those already known in the state of the art.